Posted
by
Soulskill
on Friday December 13, 2013 @02:40PM
from the bright-ideas dept.

KentuckyFC writes "Optical engineers generally build imaging systems with the best possible resolving power. The basic idea is that an imaging system focuses light into a pattern known as a point spreading function. This consists of a central region of high intensity surrounded by a concentric lobe of lower intensity light. The trick to improving resolution is narrowing and intensifying this central region while suppressing the outer lobe. Now optical engineers have turned this approach on its head by suppressing the central region so that the field intensity here is zero while intensifying the lobe. The result is a three-dimensional beam of darkness that hides any object inside it. The engineers say this region can be huge — up to 8 orders of magnitude bigger than the wavelength of the imaging light. What's more, the optics required to create it are simple and cheap: a lens consisting of concentric dielectric grooves. The team has even tested a prototype capable of hiding a 40-micrometre object in visible laser light."

You don't want this "invisibility cloak". You'd have a big fucking red lazer ring painted around you. And there'd be a big empty black patch in the middle. And some random archer is walking his dog in the next park over, and whammo! He empties his quiver into you, thinking it was a deliberate target set up. Only upon retrieving his first bloody arrow to discover that he's a murderer. Good job, assole, you ruined the archer's life.

I think they must have skipped the chapter in their basic handbook of optics called Babinet's principle. Because they just re-invented Babinet focusing.

By the way, an insightful thing to ponder here is, what happens to the light rays that were aiming for the center? (yes you can use a ray-optic basis set and still have interferrence). Well they were not in the beam! In a plane wave basis set, you would say, well all the plane waves with that K-vector were missing. Thus it's really simple to figure out how to create a dark spot. Just take an axiconically focused beam. Delete all parts of the axicon which focus in the dark region and replace them with any part of the axicon that focuses outside the dark region. Bam. that's it.

Did this myself a decade ago when I wanted arrays of dark spots in focused light. Why would I want that? I was trying to get the same effect as self fillamentation. but without non-linear effects in the media. That way I could create long arrays of ionized spots in the air, and use this to direct lighting beams.

Recently the military created a lightning weapon based on this.

But axicons and babinets prininciple this has been known for centuries.

Here's some recent articles on the topic of shaping light beams so it curves or has extended focal range or dark spots

by the way the "lightning" redirection problem we originally of interest not as a weapon but to create virtual lightning rod arrays in the air to discharge destructive lightning harmlessly. Why? well back then there had been a few great arpanet outages and people realized how vulnerable we were ebcoming to lightning stikes as we depending on the ubiquitous internet to always be able to route around problems. turned out this was a weak point. I suspect it may have become less of one now in part because optical fiber now carries stuff. But I don't know. But it was the utility companies paying for the research at the time.

"forming a pseudo non-diffrating "beam" --- which is a totally wrong way to describe this."

Hum... Why?

Imagine you took a lens, bored out the center and put another lens in the middle with a different focal length. now you shine a beam of light through this. what happens in the far field is you get two focal spots. You might have been thinking one would be a donut, but as Babinet will tell you, that donut part disappears in the focal region.

This is the crude, but easy to visualize, version of what these investigators did. One can do this more cleverly. Add more focal spots by adding concentric rings of different focii. in fact if you just make a really bad fresnel lens where you have constant curvature on each lenslet ( picture a triangle wave in glass) then you get an array of spots.

An axicon is just continuous limit of that discrete process.

it creats what looks like a beam that never difracts. but in reality is is just a bunch of separate focal spots that have merged in to a line giving that appearance.

why is this not a beam: None of the photon the form the first spot in the line actually pass through last spot. infact if you block the first spot in the "beam" it doens't stop the "beam" !

Ok, so what about smooth wavefront modulation masks such as the Cubic Phase Mask or its Zernike-based counter-part which are doing exactly the same thing. They are not creating multiple focus but rather a single longitudinally elongated focus point (which is slightly larger than the Airy disk), they are not producing beams?

On your argument that this is not a beam : you cannot use the notion of photon, nor rays, because this is pure diffraction optics... Also, if you block the beam it will change the beam st

On your argument that this is not a beam : you cannot use the notion of photon, nor rays, because this is pure diffraction optics...

Uh.... I think you need to define what you mean by "beam" if you are not going to let me use a photonic or ray optics. In a pure plane wave, which is a natural basis set for diffractive optics, there's no such thing as a "beam" at all. You have to create an ensemble and at some point when the ensemble is narrow enough you might like to call it a "beam". So now were back to "ray optics" in which each ray is a beam. I'm saying the focii are formed from different rays.

Also, if you block the beam it will change the beam structure after propagation, because you won't have a mode of the diffraction operator anymore. The effect introduced is dependent on the size of the mask and its geometrical properties with respect to the incoming beam.

axicons are continuous too, and there's nothing special about phase masks, zone plates or physcial lenses this. THere's nothing special about contiuitutiy to discontinuiuty in the lens. the fresnel lens is the missing link between all of these concepts. Just as the fresnel lens is a dicritized version of a continuous lens phase front, the mathematics can also be written as a series as well.

Ok, for beam, you can choose between typical Gaussian structures such as Legendre-Gauss (Cartesian basis) or Laguerre-Gauss (cylindrical basis). In fact the current article is just about creating symmetric high order Laguerre-Gauss modes with a binary coded phase mask. These polynomials basis are modes of the propagation operator but not solution of Maxwell Eqs.

Any light structured wave (and not wavefront, that would be a 2D surface) can be decomposed on these basis but the waves which can be decomposed on

Look, were discussing semantics. I could have started off by saying that re-imaging the exit plane of a TEM 11 mode laser would produce a donut hole in the middle of a beam like magic. But no one would but you would have a clue what were talking about. And were not really disagreeing that you can form a whole in a beam at far field. What my objective was to explain this in a very intuitive way. If you always have to run to an orthogonal function handbook to think about something then you can only solve

I think they must have skipped the chapter in their basic handbook of optics called Babinet's principle. Because they just re-invented Babinet focusing.

By the way, an insightful thing to ponder here is, what happens to the light rays that were aiming for the center? (yes you can use a ray-optic basis set and still have interferrence). Well they were not in the beam! In a plane wave basis set, you would say, well all the plane waves with that K-vector were missing. Thus it's really simple to figure out how to create a dark spot. Just take an axiconically focused beam. Delete all parts of the axicon which focus in the dark region and replace them with any part of the axicon that focuses outside the dark region. Bam. that's it.

Did this myself a decade ago when I wanted arrays of dark spots in focused light. Why would I want that? I was trying to get the same effect as self fillamentation. but without non-linear effects in the media. That way I could create long arrays of ionized spots in the air, and use this to direct lighting beams.

Recently the military created a lightning weapon based on this.

But axicons and babinets prininciple this has been known for centuries.

ray-optic basis set
plane wave basis set
plane waves with that K-vector
axiconically focused beam
parts of the axicon
self fillamentation
long arrays of ionized spots in the air

You're just spewing well formed techno-babble, aren't you? That would have sounded right at home coming out of Wesley Crusher's mouth back in the day.

Oh, and look, you're my foe, it seems. Got tired of an expert coming up to let you know you're wrong, still wrong, and will likely always be wrong?

That wasn't meant to be insulting or degrading (although, in retrospect I can see how it can be taken that way). I meant it to be humourous (See: https://xkcd.com/179/ [xkcd.com]). I apologize if my comment rubbed you the wrong way. To be fair, though, to an untrained eye, it does look like well composed pseudo-scientific doublespeak, like the pro-Mars posts from K'Breel, speaker for the Council.

Optics isn't the only field where people are "reinventing" well known concepts. It's too bad that peer review is dead, something like this would have met the big brick wall of yawn if it had been put in front of the right eyes before going to publication.

It has been easy to make objects invisible from time immemorial. Even though till about 14th century people did not really understand vision, and they thought some kind of rays emanate from the eyes that allows them to perceive it, they knew how to make objects invisible. Just turn the light off. Put it in a dark room without light, and no one can see it!

The technique here is a marginal improvement for practical purposes. There is a light in the room, still you can't see the object, objects that could be as much as 8 wavelengths of that monochromatic light, placed at the correct location. If there is any other source of light, the objects would be plainly visible due to scattering of that light. So it would be impractical to make an invisibility cloak out of it. You need to make the object disappear from all sources of light from all directions.

Though the technique is not going to lead to invisibility cloak, it is probably a great achievement to hide an object when there is one light source. Though people would think is 1 is pretty close to zero, in reality, there is a huge difference between 1 light source and no light source. It is a good great achievement, collect your brownie points scientists.

As the beam of darkness brushes across the switch for that would turn it off, then expands to envelop us all in not-light and not-dark. Other things, rogue side-effects of tortured physics nonetheless mediated by gravity, the force nature uses to keep its mistakes firewalled from each other. Cubits and qubits swirl in maelstrom like the brief screaming hiss of a black hole drifting past you on a foul orbit around the center of the Earth leaving sublimely beautiful ellipses of not-matter as its event horizon expands by degree infinitesimal, unobserved but never unnoticed but by those for whom valuable body parts have been thus punctuated, the steady rhythm of science gone mad merging with the jeering laughter of the unquiet dead.

Until we all drown in the bile in the gullet of Schrodinger's cat.

Then nature hands up a sign near the incomprehensibly entangled remnant of our solar system which says.

What this paper describes is basically using a phase-shift mask to produce a dark spot. There are established techniques for doing this in microlithography, known as "chromeless phase-shift lithography" (CPL). In lithography, one is generally trying to produce patterns of relative light and dark regions that are very small, so there's an emphasis on making thin lines, tiny contact spots, and sharp corners. But a CPL mask can produce fairly arbitrary patterns. For example, a dense checkerboard of zero and pi phase shift regions can produce a large region of region of relative darkness that could be much larger than the single point described in the paper. In practice, a large region can also be produced using conventional "chromed" lithography - a dark spot in the mask produces a dark shadow in the region behind it - but people wouldn't be so impressed by using a mask with a dark spot in it to produce a "three-dimensional beam of darkness."

Light bulbs are not actually "light bulbs" but dark absorbers. When you turn them on, they suck the dark out of the room. You can prove this by holding your hand under a "light bulb". The dark will stack up under your hand where its path to the absorber is blocked by your hand. When they quit working and turn a dark color, it's not because they burnt out, it's because they're full.

I expect this is of proper scientific merit and interest to those who understand these things, but I'm not one of them. All I get from the summary (and what little I can glean from the article this late at night) seems to amount to hiding something by not lighting it up in the first place. And I can think of easier ways to do that...

If something's already lit by another light source, surely you couldn't really actually use this like a "beam of darkness" as is suggested?